The main reason for failure in the treatment of cancer patients is resistance to conventional chemotherapeutics. One type of drug resistance is referred to as multiple resistance to cytostatics (multidrug resistance); this multiple resistance is characterized by a cross-resistance to functionally and structurally unrelated drugs such as e.g. doxorubicin, vincristine, vinblastine, colchicine and actinomycin D. The gene which is responsible for multiple resistance codes for a glycoprotein, named Pgp, which operates as an energy-dependent outflow pump for cytostatics. Some drugs from different therapeutic and chemical classes have a certain activity in partially or completely eliminating multiple resistance. This is described for the calcium channel blocker verapamil [Cancer Res. 41 1967-1972, (1981)], trifluoperazine, a calmodulin antagonist [Cancer Res. 42, 4730-4733, (1982)], quinidine, an antiarrhythmic [Cancer Res. 44, 4303-4307, (1984)], the immunosuppressor cyclosporin A [Br. J. Cancer 54, 235-238, (1986)] and rac-N-(3,4-dimethoxyphenethyl)-N-methyl-2-(2-naphthyl)-m-dithiane-2-propan amine, a calcium channel blocker [Eur. J. Med. Chem. 24, 493-496, (1989)].
Verapamil, trifluoperazine, quinidine and cyclosporin A, which are used as specific drugs in everyday therapy, are not suitable as drugs for the control of multiple resistance because of their main pharmacological activity. rac-N-(3,4-Dimethoxyphenethyl)-N-methyl-2-(2-naphthyl)-m-dithiane-2-propan amine, a relatively toxic naphthalene derivative, has a clearly lower activity vis-a-vis the novel compounds of formula I described herein and has therefore not been used in clinical trials. It has now been found that the compounds of formula I as well as rac-N-(3,4-dimethoxyphenethyl)-2-(3,4-dimethoxyphenyl)-.beta.,N-dimethyl-m -dithiane-2-propanamine [U.S. Pat. No. 4,003,914], which is known as a coronary dilator, have outstanding properties as multiple resistance-modifying drugs and can accordingly be used successfully in the therapy of malignant tumors in combination with the usual cytostatics.